In recent years, the price of energy sources has been increased due to the depletion of fossil fuels and the interest in environmental pollution has been increased. Therefore, the demand for environmentally friendly alternative energy sources has become an indispensable factor for future life. Various studies on power generation technologies such as nuclear power, solar power, wind power, and tidal power have been continuing, and power storage devices for more efficient use of such generated energy have also been attracting much attention.
In particular, as the technology development and demand for mobile devices are increasing, the demand for batteries as energy sources is rapidly increasing, and many studies on batteries meeting various demands have been conducted.
Typically, in terms of the shape of the battery, the demand for a prismatic secondary battery and a pouch type secondary battery that can be applied to products such as mobile phones having a small thickness is increased, and in terms of the material, the demand for a lithium secondary battery such as a lithium ion battery and a lithium ion polymer battery having merits such as a high energy density, a discharge voltage, and output stability is increased.
Also, the secondary battery is classified according to how to construct an electrode assembly having a structure in which a cathode, an anode, and a separator interposed between the cathode and the anode are stacked. Typically, an example of the secondary battery may include a jelly-roll type (winding type) electrode assembly having a winding structure in which long sheet-like cathodes and anodes are wound having a separator interposed therebetween, a stack type electrode assembly in which a plurality of cathodes and anodes cut in units of a predetermined size are sequentially stacked having a separator interposed therebetween or the like. In recent years, in order to solve the problems of the jelly-roll type electrode assembly and the stack type electrode assembly, as the electrode assembly having a progressive structure in which the jelly-roll type and the stack type are mixed, a stack/folding type electrode assembly having a structure in which unit cells in which cathodes and anodes in a predetermined unit are stacked having a separator interposed therebetween are sequentially wound with being located on a separation film has been developed.
In addition, the secondary battery is classified into a cylindrical battery and a prismatic battery in which the electrode assembly is embedded in a cylindrical or prismatic metal can according to the shape of the battery case, and a pouch type battery in which the electrode assembly is embedded in a pouch-like case of an aluminum laminate sheet.
In particular, recently, a pouch type battery having a structure in which a stack type or a stack/folding type electrode assembly is embedded in a pouch type battery case of an aluminum laminate sheet has attracted much attention due to low manufacturing cost, small weight, easy shape change, and the like and the usage thereof is gradually increasing.
Generally, the secondary battery is completed by manufacturing the electrode by coating and drying an electrode mixture in which an electrode active material, a conductive agent, a binder, etc. are mixed on an electrode current collector and stacking the manufactured electrode along with the separator and then embedding the electrode and the separator in the battery case along with an electrolyte solution and sealing it.
At this time, the separator is an insulating thin film having high ion transmission and mechanical strength, and has a structure including pores having a predetermined diameter. More specifically, a sheet, a non-woven fabric or the like made of olefin based polymer such as chemical-resistance and hydrophobic polypropylene, glass fiber, polyethylene or the like are used. When a solid electrolyte such as polymer is used as an electrolyte, the solid electrolyte may also serve as the separator.
However, when a size of the pores is large or porosity is high, the separator may be advantageous in movement of lithium ions, but the insulation performance exerted between the cathode and the anode may deteriorate, such that the safety of the battery may deteriorate.
On the other hand, when the size of the pore of the separator is too small or the porosity is too low, as the charge and discharge cycle of the battery is progressed, by-products generated due to the decomposition of the electrolyte solution or the like close the pores of the separator, such that the electrical performance of the battery may deteriorate.
In addition, when the solid electrolyte serves as the separator, there is an advantage that the gas generation in the battery is reduced or the safety is improved, but the impregnability property and the ion conductivity are lower than those of the liquefied electrolyte solution, such that the performance of the battery may deteriorate.
Therefore, a need for a technology capable of fundamentally solving such problems is increased.